Superconductivity is usually defined as a phenomenon wherein the electrical resistance of a material is totally vanished at a temperature below a given temperature specific to the material (the critical temperature of the material). Accordingly, the superconductive materials are drawing attention as materials for energy saving type power transmission systems and power storage systems which utilize the property of no electrical resistance, or as materials for nuclear magnetic resonance tomographic diagnostic apparatuses, electromagnetically levitated trains (linear motor cars), electric generators, superconductive magnets, accelerators, etc. In the field of electronics, the superconductive materials can be used as materials for memory elements, arithmetic elements or magnetic flux detectors by utilizing Josephson effect and magnetic flux quantization.
However, the critical temperature (Tc) of the superconductive material is very low. For example, the thus far well known superconductive materials must be cooled with very low temperature liquid helium (boiling point: 4.2.degree. K) and the cost of the coolant is very expensive. The costly cooling prevented them from being popular. Accordingly, there is demand to develop a superconductive material having a high critical temperature (Tc).